Mr Lewis said he had become aware of concerns after the ABC report which uncovered a covert plot by Australia's alt-right movement to join major political parties and influence their policy agendas from within.

It included reports of NSW Young Nationals members in Sydney attending a secret men's-only fight club set up by some of the country's most prominent alt-right nationalists.

The ABC understands NSW Young Nationals members Thomas Brasher, Clifford Jennings and Oscar Tuckfield — who were named in the Background Briefing story — are among the 18 people of interest.

NSW Nationals director Ross Cadell said he was surprised to hear ASIO was monitoring but welcomed it.

"Extremism and the politics of hate need to be shut down wherever they rise and if ASIO's involvement helps rid our party of these insurgents then it is only a good thing," he said.

Mr Cadell said an internal investigation had been running for two weeks and the 18 people identified were members of both the NSW Young Nationals and NSW Nationals, because of the party structure.

"We're gathering some social media content and history of that and also contacting them with a number of questions seeking their reason for joining and their motivation," Mr Cadell said.

Nationals keen on 'natural justice'

The NSW Nationals called a special meeting for Monday of their Constitution and Ethics Committee.